Hi Kittybiccy,
Is the error in the CMS or on one of the frontend pages?
"Undefine Index" means PHP tried to get a value from an array and it wasn't there. Usually $_SERVER['HTTP_USER_AGENT'] is set to the name of the users browser. Usually something like the following. Your browser sends that with every page request to identify which kind of browser it is.
Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36
So either the CMS, or a CMS plugin, or one of your frontend pages is trying to access $_SERVER['HTTP_USER_AGENT'] and it's not defined. Maybe because someone's browser isn't sending it or because a script is being run from the server shell such as from a cron job.
You need to figure out which page it's on to further isolate the problem.
Dave Edis - Senior Developer
interactivetools.com